A Yono-class midget submarine, described by one expert as "swimmer-delivery vehicles," may be an increasing threat.

Don't let the funny, politically incorrect name fool you: Midget subs are a real threat. In the hands of North Korean and Iranian navies, these small vessels make good platforms for ambushesâ€”and the U.S. Navy is clearly ready to hone its anti-submarine skills.

War tensions have been high since last week's announcement by the South Korean government that a 60-foot North Korean submarine fired a torpedo that sank a South Korean corvette and killed 46 sailors at the end of March.

The South Koreans stated that a Yeono-class (alternatively spelled "Yono") midget submarine fired the torpedo in March. They also field a larger midget submarine, the Sang-O, that fits 15 sailors. At least one of these subs was also on patrol when the attack happened, according to an international team of investigators looking into the incident with South Korea. The attack occurred in 150 feet of water, enough room for the midget submarine to maneuver.

Any sub that weighs less than 150 tons is called a midget. They can't travel too far on their own, and depend on support vessels to extend their range. In shallow water, where sonar returns are cluttered, they can prove quiet and sneaky. Often this means they can lay mines or insert commandos on beaches.

According to statements by South Korea, attacks from midget subs can also include torpedoes. Iran is known to operate midget subs, and after buying a handful from North Korea, it is believed to be making its own.

A civilian ship hired to dredge the area of the attack found remains of what the government labeled a CHT-02D torpedo, made in North Korea. That torpedo would have a big enough warhead â€” 250 kilograms â€” to destroy the corvette. Government reports state that the ship's sonar did not detect the submarine or the torpedo.

That's what concerns the U.S. Navy.

Two things heighten the risk of a similar ambush by midget submarines against U.S. ships: the complex sonar picture of shallow water where these small subs can operate, and a postâ€“Cold War decrease in anti-submarine training.

"Instead of a large number of Soviet nuclear-powered submarines on the open ocean, advanced conventional submarines operating in the littorals have emerged as the most serious threat to U.S. forwardly deployed forces, military sealift and merchant shipping," Milan Vego, professor of operations at the Joint Military Operations Department at the Naval War College, wrote in a recent piece for Armed Forces Journal.

This week the Pentagon announced it would step up its anti-submarine training, engaging in exercises with South Korea. The decision is "a result of the findings of this recent incident," Pentagon spokesman Bryan Whitman told reporters. But crash courses in sub hunting may not help much; professionals admit it's an art as much as a science.

The United States' sub-hunting abilities have atrophied since the Soviet Union dissolved. One obstacle to revamping anti-submarine training is bringing it out of simulators and into the real world. It takes a lot of effort to conduct a real sub hunt, but these skills need to be continuously honed. "The skills for successful conduct of anti-submarine warfare (ASW) must be maintained; otherwise, they will quickly atrophy," Vego warns.

The Navy has done a better job spending money on technology that can locate submarines. During the Cold War, permanent networks of sensors on the sea floor helped keep tabs on Soviet submarines. Similar networks have not been established or upgraded for use in new hotspots. "Undersea surveillance systems developed during the Cold War have limited effectiveness today," Vego says.

It appears the South Koreans share that lethargy, but South Korean officials now say a permanent snooping system will be installed. South Korean Lt. Gen. Park Jung-e said at a media briefing that "our plan is to reinforce submarine measures by establishing a submarine detection system in areas that are vulnerable."

The United States is also fielding a deployable piece of underwater detection technology, called the Advanced Deployable System (ADS), that is built for shallow-water emergencies. The system proposes to use expendable, battery-powered passive acoustic arrays that are connected with fiberoptic cables. The system will be integrated into the Navy's much-delayed but recently commissioned Littoral Combat Ship.

...what makes this number station significant is the proximity in timing to the disappearance of the San-Ho class submarines, as well as the digital transmission. It is important to point out that the Sang-Ho submarines in question were effectively purpose built for covert insertion of small teams ofNorth Korean Maritime Commandos who are part of North Koreaâ€™s much lauded Special Operations Force. So at this moment there is a covert North Korean force apparently heading for South Korea.

The Sang-Ho submarines can carry as many as 30 commandos per submarines giving the current submarines a combined force of around 60 troops. Those 60 troops are specially trained to carry out behind enemy lines covert actions such as sabotage and assassinations. So in this case the numbers station was most likely broadcasting a special code to North Korean operatives to announce the impending arrival of the covert operations teams.